Obtained inheritance east of Jordan on condition of helping to conquer Canaan

Numbers 32:1-33 - And Moses gave unto them, even to the children of Gad, and to the children of Reuben, and unto half the tribe of Manasseh the son of Joseph, the kingdom of Sihon king of the Amorites, and the kingdom of Og king of Bashan, the land, with the cities thereof in the coasts, even the cities of the country round about.

Deuteronomy 3:18-20 - Until the Lord have given rest unto your brethren, as well as unto you, and until they also possess the land which the Lord your God hath given them beyond Jordan: and then shall ye return every man unto his possession, which I have given you.